1. Field of the Invention
The present invention relates to a feldspathic porcelain for a dental restoration comprising one or more various leucite phases and the method of production thereof. More particularly, this invention relates to feldspathic porcelains which are machinable into a variety of dental articles, including dental restorations. The materials of the present invention are also especially useful for fabrication of dental restorations using CAD/CAM technology.
2. Brief Description of the Related Art
Making dental restorations is important in many situations. As human teeth grow older, they are subjected to naturally occurring breakdowns such as decay and wear. The decay of teeth is normally corrected through semi-permanent means such as fillings and the like. However, after many years, tooth decay can progress to point where restoration of the tooth through an inlay, onlay, or crown becomes necessary. Dental restorations are also important in the situation where a tooth or several teeth have been chipped, cracked and/or broken because of an accident or an act which causes a blow to the mouth. When this situation arises, the patient requires relief from the associated pain, but also desires to have the injured tooth or teeth to be restored to their appearance before the injurious event. In this situation, the dental restoration is likely to be in the form of an inlay, onlay, or crown.
Conventional methods of preparing restorations is very often time consuming for both the patient and the dentist and there is a degree of imperfection in matching the restoration to the patient's other teeth. Generally, the entire process of matching and fabricating the restoration requires more than one visit to the dentist's office. Usually, during the first visit, the dentist prepares the tooth or teeth for restoration and also fits the prepared tooth/teeth with a temporary substitute until the restoration is completed. At this first visit, an impression is taken and a working model is fabricated with individual teeth separated and indexed to be able to be precisely reassembled. Once the impression is fully prepared by the dentist, the model is then sent to a dental technician to fabricate the final dental restoration. When the dentist receives the restoration back from the technician, a fitting is performed in which the substitute restoration is removed and the final restoration is adjusted and permanently placed in the patient's mouth.
Porcelains have become the preferred material to construct a dental restoration. Porcelain dental restorations, such as crowns, bridges, and the like are highly favored because the porcelains provide strength, wear resistance, and good aesthetics. Older porcelain restorations generally comprise at least one porcelain layer on a metal framework. Newer restorations, however, generally comprise a ceramic core in place of the traditional metal, with at least one additional porcelain layer. These are commonly referred to as "all-ceramic" systems, and provide even better aesthetics than the metal-porcelain systems.
Formation of either porcelain-to-metal or all-ceramic restorations requires consideration of a number of factors, including the fusion temperature of the various layers, the size and distribution of the crystalline phase, and the materials' coefficients of thermal expansion (hereinafter CTE).
Currently available porcelain dental restorations may contain a leucite component. Leucite is a crystalline potassium aluminum silicate (K.sub.2 O.Al.sub.2 O.sub.3.4SiO.sub.2) which is used for reinforcement of feldspathic dental porcelains. At room temperature leucite ordinarily has a tetragonal configuration, and when the leucite exists in this form, it is referred to as "low leucite". The use of tetragonal leucite for reinforcement of feldspathic dental porcelains is well known and described in U.S. Pat. No. 4,604,366 to Kacicz et al., and U.S. Pat. No. 4,798,536 to Katz, all of which are incorporated by reference in their entirety.
At room temperature, leucite normally exists in the tetragonal configuration because it is the thermodynamically stable configuration of leucite at this temperature. When tetragonal leucite is heated to about 625.degree. C. it undergoes a reversible transformation to a cubic polymorph, with a concomitant volume change of 1.2%. The cubic phase of leucite is known as "high leucite." Upon cooling to room temperature, the cubic leucite crystals revert to the more stable tetragonal polymorph.
Pollucite is a cubic compound similar to leucite and has the stoichiometric composition of Cs.sub.2 O.Al.sub.2 O.sub.3.4SiO.sub.2. U.S. Pat. No. 3,723,140 to Beall and Rittler disclose a method for forming highly crystalline glass-ceramic bodies comprising a uniform dispersion of a fine-grained pollucite phase. The approach is based on the use of self-nucleating compositions having a high alumina-to-alkali ratio, wherein the ratio of Al.sub.2 O.sub.3 to the sum of RO+R.sub.2 O is greater than 1.2. This ratio presumably results in the formation of submicroscopic nuclei of mullite upon which the pollucite crystals subsequently grow. However, the compositions disclosed in Beall and Rittler appear to be too refractory for use as dental restoration materials.
Other stable forms of cubic leucite have also been reported, wherein only a fraction of the potassium has been replaced by cesium, rubidium, and the like. For example, the formation of porcelains comprising cubic leucite by the volume crystallization of glasses containing about 2 mole % of CsO.sub.2 has been reported by C. Hahn and K. Teuchert in "Importance of the Glass Ceramic System K.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 in Dental Porcelain", Ceramic Forum International/Ber. Dt. Keram. Ges 57 (1980) No. 9-10, pp. 208-214, and by A. Prasad and T. K. Vaidyanathan in "Crystallization of Cubic Leucite by Composition Additives", 19.sup.th annual session, American association of Dental Research, Mar. 9, 1990.
Feldspathic dental porcelains have superior aesthetic characteristics and have a wide use in a variety of all-ceramic restorations. Because of these qualities and others, feldspathic dental porcelains are widely used in all-ceramic dental restorations. Unfortunately, the currently available high-strength feldspathic dental porcelains are not readily machinable even when diamond tooling is the means for shaping these porcelains. Commercially available machining devices such as the Celay.TM. system available from Mikrona Technologie, Spreitenbach, Switzerland and the CEREC.TM. system manufactured by Siemens Dental Corp., Benshein, Germany, are equipped mostly with diamond tooling such as disks and/or end-mills and are sold to dental laboratories and dentists. These devices are compact in size and are quite sophisticated but have limited ability in the machining of high strength ceramics. Consequently, high strength feldspathic dental porcelains can not be used in machining devices that are employed in combination with CAD/CAM technology.
The introduction of CAD/CAM technology to the dental field has brought great enthusiasm and numerous potential applications, and is described, for example, in U.S. Pat. No. 5,549,476 to Stern, U.S. Pat. No. 5,527,182 to Willoughby, and U.S. Pat. No. 5,775,912 to Panzera et al., all of which are herein incorporated by reference in their entirety. CAD/CAM technology refers to an integrated system of computer-aided design and computer-aided manufacturing. Recently, computer reconstruction of dental restorations became commercially feasible. CAD/CAM devices are commercially availably from Siemens AG (CEREC.TM. system) and Elephant Holding BV (Cicero.TM. system); also a copy-milling system (Celay.TM.) is available from Mikrona Technologie AG. In general, CAD/CAM systems have an optical contact digitizer which generates a computer-read signal directed to the shape of the restoration. With respect to dental restorations, commercially available CAD/CAM devices optically or mechanically read tooth areas in conjunction with dental reconstruction. This technology digitizes information from the patient's mouth or from a model of the patient's mouth using optical scanning to create a customized restoration. The use of optical impression systems, however, greatly reduces the amount of time involved in preparing a dental restoration as compared to conventional methods, but several drawbacks exist.
Generally, the accuracy of dental CAD/CAM systems is about 80-100 microns, and dental CAD/CAM devices have been used only to create inlays, onlays, and in more select instances crowns. Additionally, current machinable dental ceramics for use in CAD/CAM devices are either limited in the available shades, translucency or the ceramics require post-machining sintering/infiltration. In the dental field, at least two approaches to CAD/CAM technology are known. One approach, used in the Cerec and Celay systems, involves the use of presintered blanks and the other method used in the Procera system available from Noblepharma Inc., Goteborg, Sweden employs green bodies of ceramic material such as alumina. In the first approach, all the steps including the machining are completed in the dental office and/or the dental laboratory. The second method involves the exchange of materials and data with the central processing center where the actual machining and sintering are performed. Currently, the availability of materials for commercial CAD/CAM devices utilizing pre-sintered blanks is substantially limited to a fluormica-based glass ceramic available under the trade name Dicor MGC (commercially available from Dicor,Dentsply international, York, Pa.; porous alumina and spinel blanks available under the trade name Vita In-Ceram and sanidine-based porcelain available under the trade name Vita Mark II (both commercially available from Vita Zahnfabrick, Bad Sachingen). Machinability in Dicor MCG is associated with cleavage of mica grains. However this material has the substantial drawback that it can only be produced in limited shades. Soft sintered Vita In-Ceram alumina and spinel require the subsequent glass infiltration step following machining of the blanks and are very fragile before infiltration. The use of the Vita Mark II material suffers from the disadvantage that these blanks contain a sanidine phase which renders this material very opaque.
Despite their advantages, high-strength feldspathic porcelains have not been able to be fully utilized in the dental arts because of the associated machinability limitations. Commercially available high-strength feldspathic porcelains, such as OPC.RTM. (available from Jeneric/Pentron, Wallingford, Conn.), are currently used for hot pressing rather than machining cores for all-ceramic restorations, including crowns, inlays and onlays. These porcelains comprise 40% to 50% of a leucite phase as the reinforcement. To enhance machinability of feldspathic porcelains, the grain size of their leucite constituent should be substantially reduced and its distribution should be homogeneous throughout the glass matrix of the porcelain. However, currently, no commercial techniques are available to form in these dental porcelains the sufficient volume fraction of the leucite phase as fine-grained and uniformly dispersed as required to assure a level of machinability necessary to fabricate the complex shape of a dental restoration in an aesthetic manner.
The conventional crystallization of leucite in feldspathic glasses should be carried out at sufficiently-high temperatures, e.g. .gtoreq.980.degree. C., to avoid crystallization of "parasitic" phases such as sanidine or feldspar. Consequently, the conventional crystallization methods may require higher crystallization temperatures to avoid formation of "parasitic" phases. However, the crystallization at these higher temperatures favors crystallization of coarser leucite particles. The presence of these coarser leucite particles inhibits the porcelains from being easily machined or used in CAD/CAM devices.
Recently, as described in pending Application No. 08/960,684 filed Oct. 30, 1997 to Denry, now U.S. Pat. No. 5,994,246, which is hereby incorporated by reference, porcelains comprising fine and uniformly dispersed cubic leucite may be manufactured by the ion-exchange of the starting glass frit with a metal salt such as rubidium nitrate. This application does not specifically teach how to use the ion-exchange method disclosed therein to produce machinable ceramics, more specifically CAD/CAM blanks.
Accordingly, there is a need for a method to produce a high-strength feldspathic dental porcelain which is readily machinable and which may be used with CAD/CAM devices.